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Jendruchová K, Gaikwad S, Poncová K, Gunišová S, Valášek LS, Hinnebusch AG. Differential effects of 40S ribosome recycling factors on reinitiation at regulatory uORFs in GCN4 mRNA are not dictated by their roles in bulk 40S recycling. Commun Biol 2024; 7:1083. [PMID: 39232119 PMCID: PMC11375166 DOI: 10.1038/s42003-024-06761-x] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/06/2024] [Accepted: 08/21/2024] [Indexed: 09/06/2024] Open
Abstract
Recycling of 40S ribosomal subunits following translation termination, entailing release of deacylated tRNA and dissociation of the empty 40S from mRNA, involves yeast Tma20/Tma22 heterodimer and Tma64, counterparts of mammalian MCTS1/DENR and eIF2D. MCTS1/DENR enhance reinitiation (REI) at short upstream open reading frames (uORFs) harboring penultimate codons that confer heightened dependence on these factors in bulk 40S recycling. Tma factors, by contrast, inhibited REI at particular uORFs in extracts; however, their roles at regulatory uORFs in vivo were unknown. We examined effects of eliminating Tma proteins on REI at regulatory uORFs mediating translational control of GCN4 optimized for either promoting (uORF1) or preventing (uORF4) REI. We found that the Tma proteins generally impede REI at native uORF4 and its variants equipped with various penultimate codons regardless of their Tma-dependence in bulk recycling. The Tma factors have no effect on REI at native uORF1 and equipping it with Tma-hyperdependent penultimate codons generally did not confer Tma-dependent REI; nor did converting the uORFs to AUG-stop elements. Thus, effects of the Tma proteins vary depending on the REI potential of the uORF and penultimate codon, but unlike in mammals, are not principally dictated by the Tma-dependence of the codon in bulk 40S recycling.
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Affiliation(s)
- Kristína Jendruchová
- Laboratory of Regulation of Gene Expression, Institute of Microbiology of the Czech Academy of Sciences, Videnska 1083, 142 20, Prague, Czech Republic
- Faculty of Science, Charles University, Albertov 6, 128 00, Prague, Czech Republic
| | - Swati Gaikwad
- Division of Molecular and Cellular Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD, 20892, USA
| | - Kristýna Poncová
- Laboratory of Regulation of Gene Expression, Institute of Microbiology of the Czech Academy of Sciences, Videnska 1083, 142 20, Prague, Czech Republic
| | - Stanislava Gunišová
- Laboratory of Regulation of Gene Expression, Institute of Microbiology of the Czech Academy of Sciences, Videnska 1083, 142 20, Prague, Czech Republic
| | - Leoš Shivaya Valášek
- Laboratory of Regulation of Gene Expression, Institute of Microbiology of the Czech Academy of Sciences, Videnska 1083, 142 20, Prague, Czech Republic.
| | - Alan G Hinnebusch
- Division of Molecular and Cellular Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD, 20892, USA.
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Gayen A, Alone P. eIF2β zinc-binding domain interacts with the eIF2γ subunit through the guanine nucleotide binding interface to promote Met-tRNAiMet binding. Biosci Rep 2024; 44:BSR20240438. [PMID: 38873976 PMCID: PMC11230868 DOI: 10.1042/bsr20240438] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/12/2024] [Revised: 06/09/2024] [Accepted: 06/13/2024] [Indexed: 06/15/2024] Open
Abstract
The heterotrimeric eIF2 complex consists of a core eIF2γ subunit to which binds eIF2α and eIF2β subunits and plays an important role in delivering the Met-tRNAiMet to the 40S ribosome and start codon selection. The intricacies of eIF2β-γ interaction in promoting Met-tRNAiMet binding are not clearly understood. Previously, the zinc-binding domain (ZBD) eIF2βS264Y mutation was reported to cause Met-tRNAiMet binding defect due to the intrinsic GTPase activity. We showed that the eIF2βS264Y mutation has eIF2β-γ interaction defect. Consistently, the eIF2βT238A intragenic suppressor mutation restored the eIF2β-γ and Met-tRNAiMet binding. The eIF2β-ZBD residues Asn252Asp and Arg253Ala mutation caused Met-tRNAiMet binding defect that was partially rescued by the eIF2βT238A mutation, suggesting the eIF2β-ZBD modulates Met-tRNAiMet binding. The suppressor mutation rescued the translation initiation fidelity defect of the eIF2γN135D SW-I mutation and eIF2βF217A/Q221A double mutation in the HTH domain. The eIF2βT238A suppressor mutation could not rescue the eIF2β binding defect of the eIF2γV281K mutation; however, combining the eIF2βS264Y mutation with the eIF2γV281K mutation was lethal. In addition to the previously known interaction of eIF2β with the eIF2γ subunit via its α1-helix, the eIF2β-ZBD also interacts with the eIF2γ subunit via guanine nucleotide-binding interface; thus, the eIF2β-γ interacts via two distinct binding sites.
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Affiliation(s)
- Aranyadip Gayen
- School of Biological Sciences, National Institute of Science Education and Research Bhubaneswar, P.O Jatni, Khurda 752050, India
- Homi Bhabha National Institute (HBNI), Anushakti Nagar, Mumbai 400094, India
| | - Pankaj V. Alone
- School of Biological Sciences, National Institute of Science Education and Research Bhubaneswar, P.O Jatni, Khurda 752050, India
- Homi Bhabha National Institute (HBNI), Anushakti Nagar, Mumbai 400094, India
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3
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Khan D, Fox PL. Host-like RNA Elements Regulate Virus Translation. Viruses 2024; 16:468. [PMID: 38543832 PMCID: PMC10976276 DOI: 10.3390/v16030468] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/07/2024] [Revised: 03/14/2024] [Accepted: 03/17/2024] [Indexed: 04/01/2024] Open
Abstract
Viruses are obligate, intracellular parasites that co-opt host cell machineries for propagation. Critical among these machineries are those that translate RNA into protein and their mechanisms of control. Most regulatory mechanisms effectuate their activity by targeting sequence or structural features at the RNA termini, i.e., at the 5' or 3' ends, including the untranslated regions (UTRs). Translation of most eukaryotic mRNAs is initiated by 5' cap-dependent scanning. In contrast, many viruses initiate translation at internal RNA regions at internal ribosome entry sites (IRESs). Eukaryotic mRNAs often contain upstream open reading frames (uORFs) that permit condition-dependent control of downstream major ORFs. To offset genome compression and increase coding capacity, some viruses take advantage of out-of-frame overlapping uORFs (oORFs). Lacking the essential machinery of protein synthesis, for example, ribosomes and other translation factors, all viruses utilize the host apparatus to generate virus protein. In addition, some viruses exhibit RNA elements that bind host regulatory factors that are not essential components of the translation machinery. SARS-CoV-2 is a paradigm example of a virus taking advantage of multiple features of eukaryotic host translation control: the virus mimics the established human GAIT regulatory element and co-opts four host aminoacyl tRNA synthetases to form a stimulatory binding complex. Utilizing discontinuous transcription, the elements are present and identical in all SARS-CoV-2 subgenomic RNAs (and the genomic RNA). Thus, the virus exhibits a post-transcriptional regulon that improves upon analogous eukaryotic regulons, in which a family of functionally related mRNA targets contain elements that are structurally similar but lacking sequence identity. This "thrifty" virus strategy can be exploited against the virus since targeting the element can suppress the expression of all subgenomic RNAs as well as the genomic RNA. Other 3' end viral elements include 3'-cap-independent translation elements (3'-CITEs) and 3'-tRNA-like structures. Elucidation of virus translation control elements, their binding proteins, and their mechanisms can lead to novel therapeutic approaches to reduce virus replication and pathogenicity.
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Affiliation(s)
- Debjit Khan
- Department of Cardiovascular and Metabolic Sciences, Lerner Research Institute, Cleveland Clinic, Cleveland, OH 44195, USA
| | - Paul L. Fox
- Department of Cardiovascular and Metabolic Sciences, Lerner Research Institute, Cleveland Clinic, Cleveland, OH 44195, USA
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Jendruchová K, Gaikwad S, Poncová K, Gunišová S, Valášek LS, Hinnebusch AG. Impacts of yeast Tma20/MCTS1, Tma22/DENR and Tma64/eIF2D on translation reinitiation and ribosome recycling. BIORXIV : THE PREPRINT SERVER FOR BIOLOGY 2024:2024.03.06.583729. [PMID: 38903097 PMCID: PMC11188067 DOI: 10.1101/2024.03.06.583729] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/22/2024]
Abstract
Recycling of 40S ribosomal subunits following translation termination, entailing release of deacylated tRNA and dissociation of the empty 40S subunit from mRNA, involves yeast Tma20/Tma22 heterodimer and Tma64, counterparts of mammalian MCTS1/DENR and eIF2D. MCTS1/DENR enhance reinitiation at short upstream open reading frames (uORFs) harboring penultimate codons that confer dependence on these factors in bulk 40S recycling. Tma factors, by contrast, inhibited reinitiation at particular uORFs in extracts; however, their roles at regulatory uORFs in vivo were unknown. We examined effects of eliminating Tma proteins on reinitiation at regulatory uORFs mediating translational control of GCN4 optimized for either promoting (uORF1) or preventing (uORF4) reinitiation. We found that the Tma proteins generally impede reinitiation at native uORF4 and uORF4 variants equipped with various penultimate codons regardless of their Tma-dependence in bulk recycling. The Tma factors have no effect on reinitiation at native uORF1, and equipping uORF1 with Tma-dependent penultimate codons generally did not confer Tma-dependent reinitiation; nor did converting the uORFs to AUG-stop elements. Thus, effects of the Tma proteins vary depending on the reinitiation potential of the uORF and the penultimate codon, but unlike in mammals, are not principally dictated by the Tma-dependence of the codon in bulk 40S recycling.
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Affiliation(s)
- Kristína Jendruchová
- Laboratory of Regulation of Gene Expression, Institute of Microbiology of the Czech Academy of Sciences, Videnska 1083, 142 20, Prague, the Czech Republic
- Faculty of Science, Charles University, Albertov 6, 128 00 Prague, Czech Republic
| | - Swati Gaikwad
- Divsion of Molecular and Cellular Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, USA
| | - Kristýna Poncová
- Laboratory of Regulation of Gene Expression, Institute of Microbiology of the Czech Academy of Sciences, Videnska 1083, 142 20, Prague, the Czech Republic
| | - Stanislava Gunišová
- Laboratory of Regulation of Gene Expression, Institute of Microbiology of the Czech Academy of Sciences, Videnska 1083, 142 20, Prague, the Czech Republic
| | - Leoš Shivaya Valášek
- Laboratory of Regulation of Gene Expression, Institute of Microbiology of the Czech Academy of Sciences, Videnska 1083, 142 20, Prague, the Czech Republic
| | - Alan G Hinnebusch
- Divsion of Molecular and Cellular Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, USA
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Manske F, Ogoniak L, Jürgens L, Grundmann N, Makałowski W, Wethmar K. The new uORFdb: integrating literature, sequence, and variation data in a central hub for uORF research. Nucleic Acids Res 2022; 51:D328-D336. [PMID: 36305828 PMCID: PMC9825577 DOI: 10.1093/nar/gkac899] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2022] [Revised: 09/28/2022] [Accepted: 10/03/2022] [Indexed: 02/07/2023] Open
Abstract
Upstream open reading frames (uORFs) are initiated by AUG or near-cognate start codons and have been identified in the transcript leader sequences of the majority of eukaryotic transcripts. Functionally, uORFs are implicated in downstream translational regulation of the main protein coding sequence and may serve as a source of non-canonical peptides. Genetic defects in uORF sequences have been linked to the development of various diseases, including cancer. To simplify uORF-related research, the initial release of uORFdb in 2014 provided a comprehensive and manually curated collection of uORF-related literature. Here, we present an updated sequence-based version of uORFdb, accessible at https://www.bioinformatics.uni-muenster.de/tools/uorfdb. The new uORFdb enables users to directly access sequence information, graphical displays, and genetic variation data for over 2.4 million human uORFs. It also includes sequence data of >4.2 million uORFs in 12 additional species. Multiple uORFs can be displayed in transcript- and reading-frame-specific models to visualize the translational context. A variety of filters, sequence-related information, and links to external resources (UCSC Genome Browser, dbSNP, ClinVar) facilitate immediate in-depth analysis of individual uORFs. The database also contains uORF-related somatic variation data obtained from whole-genome sequencing (WGS) analyses of 677 cancer samples collected by the TCGA consortium.
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Affiliation(s)
- Felix Manske
- Institute of Bioinformatics, University of Münster, Münster 48149, Germany
| | - Lynn Ogoniak
- Institute of Bioinformatics, University of Münster, Münster 48149, Germany
| | - Lara Jürgens
- Department of Medicine A, Hematology, Oncology, Hemostaseology and Pneumology, University Hospital Münster, Münster 48149, Germany
| | - Norbert Grundmann
- Institute of Bioinformatics, University of Münster, Münster 48149, Germany
| | - Wojciech Makałowski
- Correspondence may also be addressed to Wojciech Makałowski. Tel: +49 2518353006;
| | - Klaus Wethmar
- To whom correspondence should be addressed. Tel: +49 2518347587; Fax: +49 2518347588;
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Antony A C, Ram AK, Dutta K, Alone PV. Ribosomal mutation in helix 32 of 18S rRNA alters fidelity of eukaryotic translation start site selection. FEBS Lett 2019; 593:852-867. [PMID: 30900251 DOI: 10.1002/1873-3468.13369] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/01/2018] [Revised: 03/14/2019] [Accepted: 03/18/2019] [Indexed: 11/09/2022]
Abstract
The 40S ribosome plays a critical role in start codon selection. To gain insights into the role of its 18S rRNA in start codon selection, a suppressor screen was performed that suppressed the preferential UUG start codon recognition (Suppressor of initiation codon: Sui- phenotype) associated with the eIF5G31R mutant. The C1209U mutation in helix h32 of 18S rRNA was found to suppress the Sui- and Gcn- (failure to derepress GCN4 expression) phenotype of the eIF5G31R mutant. The C1209U mutation suppressed Sui- and Gcd- (constitutive derepression of GCN4 expression) phenotype of eIF2βS264Y , eIF1K60E , and eIF1A-ΔC mutation. We propose that the C1209U mutation in 40S ribosomal may perturb the premature head rotation in 'Closed/PIN ' state and enhance the stringency of translation start site selection.
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Affiliation(s)
- Charles Antony A
- School of Biological Sciences, National Institute of Science Education and Research, Bhubaneswar, India.,Homi Bhabha National Institute (HBNI), Mumbai, India
| | - Anup Kumar Ram
- School of Biological Sciences, National Institute of Science Education and Research, Bhubaneswar, India.,Homi Bhabha National Institute (HBNI), Mumbai, India
| | - Kalloly Dutta
- School of Biological Sciences, National Institute of Science Education and Research, Bhubaneswar, India.,Homi Bhabha National Institute (HBNI), Mumbai, India
| | - Pankaj V Alone
- School of Biological Sciences, National Institute of Science Education and Research, Bhubaneswar, India.,Homi Bhabha National Institute (HBNI), Mumbai, India
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7
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Crawford RA, Pavitt GD. Translational regulation in response to stress in Saccharomyces cerevisiae. Yeast 2018; 36:5-21. [PMID: 30019452 PMCID: PMC6492140 DOI: 10.1002/yea.3349] [Citation(s) in RCA: 55] [Impact Index Per Article: 9.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/03/2018] [Revised: 06/08/2018] [Accepted: 06/25/2018] [Indexed: 12/19/2022] Open
Abstract
The budding yeast Saccharomyces cerevisiae must dynamically alter the composition of its proteome in order to respond to diverse stresses. The reprogramming of gene expression during stress typically involves initial global repression of protein synthesis, accompanied by the activation of stress‐responsive mRNAs through both translational and transcriptional responses. The ability of specific mRNAs to counter the global translational repression is therefore crucial to the overall response to stress. Here we summarize the major repressive mechanisms and discuss mechanisms of translational activation in response to different stresses in S. cerevisiae. Taken together, a wide range of studies indicate that multiple elements act in concert to bring about appropriate translational responses. These include regulatory elements within mRNAs, altered mRNA interactions with RNA‐binding proteins and the specialization of ribosomes that each contribute towards regulating protein expression to suit the changing environmental conditions.
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Affiliation(s)
- Robert A Crawford
- Division of Molecular and Cellular Function, School of Biological Sciences, Faculty of Biology, Medicine and Health, Manchester Academic Health Science Centre, The University of Manchester, Michael Smith Building, Dover Street, Manchester, M13 9PT, UK
| | - Graham D Pavitt
- Division of Molecular and Cellular Function, School of Biological Sciences, Faculty of Biology, Medicine and Health, Manchester Academic Health Science Centre, The University of Manchester, Michael Smith Building, Dover Street, Manchester, M13 9PT, UK
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8
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Gunišová S, Hronová V, Mohammad MP, Hinnebusch AG, Valášek LS. Please do not recycle! Translation reinitiation in microbes and higher eukaryotes. FEMS Microbiol Rev 2018; 42:165-192. [PMID: 29281028 PMCID: PMC5972666 DOI: 10.1093/femsre/fux059] [Citation(s) in RCA: 64] [Impact Index Per Article: 10.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/25/2017] [Accepted: 12/20/2017] [Indexed: 12/14/2022] Open
Abstract
Protein production must be strictly controlled at its beginning and end to synthesize a polypeptide that faithfully copies genetic information carried in the encoding mRNA. In contrast to viruses and prokaryotes, the majority of mRNAs in eukaryotes contain only one coding sequence, resulting in production of a single protein. There are, however, many exceptional mRNAs that either carry short open reading frames upstream of the main coding sequence (uORFs) or even contain multiple long ORFs. A wide variety of mechanisms have evolved in microbes and higher eukaryotes to prevent recycling of some or all translational components upon termination of the first translated ORF in such mRNAs and thereby enable subsequent translation of the next uORF or downstream coding sequence. These specialized reinitiation mechanisms are often regulated to couple translation of the downstream ORF to various stimuli. Here we review all known instances of both short uORF-mediated and long ORF-mediated reinitiation and present our current understanding of the underlying molecular mechanisms of these intriguing modes of translational control.
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Affiliation(s)
- Stanislava Gunišová
- Laboratory of Regulation of Gene Expression, Institute of Microbiology ASCR, Videnska 1083, Prague, 142 20, the Czech Republic
| | - Vladislava Hronová
- Laboratory of Regulation of Gene Expression, Institute of Microbiology ASCR, Videnska 1083, Prague, 142 20, the Czech Republic
| | - Mahabub Pasha Mohammad
- Laboratory of Regulation of Gene Expression, Institute of Microbiology ASCR, Videnska 1083, Prague, 142 20, the Czech Republic
| | - Alan G Hinnebusch
- Laboratory of Gene Regulation and Development, Eunice Kennedy Shriver National Institute of Child Health and Human Development, NIH, Bethesda, MD 20892, USA
| | - Leoš Shivaya Valášek
- Laboratory of Regulation of Gene Expression, Institute of Microbiology ASCR, Videnska 1083, Prague, 142 20, the Czech Republic
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9
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Hronová V, Mohammad MP, Wagner S, Pánek J, Gunišová S, Zeman J, Poncová K, Valášek LS. Does eIF3 promote reinitiation after translation of short upstream ORFs also in mammalian cells? RNA Biol 2017; 14:1660-1667. [PMID: 28745933 DOI: 10.1080/15476286.2017.1353863] [Citation(s) in RCA: 24] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
Reinitiation after translation of short upstream ORFs (uORFs) represents one of the means of regulation of gene expression on the mRNA-specific level in response to changing environmental conditions. Over the years it has been shown-mainly in budding yeast-that its efficiency depends on cis-acting features occurring in sequences flanking reinitiation-permissive uORFs, the nature of their coding sequences, as well as protein factors acting in trans. We earlier demonstrated that the first two uORFs from the reinitiation-regulated yeast GCN4 mRNA leader carry specific structural elements in their 5' sequences that interact with the translation initiation factor eIF3 to prevent full ribosomal recycling post their translation. Actually, this interaction turned out to be instrumental in stabilizing the mRNA·40S post-termination complex, which is thus capable to eventually resume scanning and reinitiate on the next AUG start site downstream. Recently, we also provided important in vivo evidence strongly supporting the long-standing idea that to stimulate reinitiation, eIF3 has to remain bound to ribosomes elongating these uORFs until their stop codon has been reached. Here we examined the importance of eIF3 and sequences flanking uORF1 of the human functional homolog of yeast GCN4, ATF4, in stimulation of efficient reinitiation. We revealed that the molecular basis of the reinitiation mechanism is conserved between yeasts and humans.
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Affiliation(s)
- Vladislava Hronová
- a Laboratory of Regulation of Gene Expression , Institute of Microbiology ASCR , Videnska, Prague , the Czech Republic.,b Department of Genetics and Microbiology, Faculty of Science , Charles University in Prague , Vinicna, Prague , the Czech Republic
| | - Mahabub Pasha Mohammad
- a Laboratory of Regulation of Gene Expression , Institute of Microbiology ASCR , Videnska, Prague , the Czech Republic
| | - Susan Wagner
- a Laboratory of Regulation of Gene Expression , Institute of Microbiology ASCR , Videnska, Prague , the Czech Republic
| | - Josef Pánek
- c Laboratory of Bioinformatics , Institute of Microbiology ASCR , Videnska, Prague , the Czech Republic
| | - Stanislava Gunišová
- a Laboratory of Regulation of Gene Expression , Institute of Microbiology ASCR , Videnska, Prague , the Czech Republic
| | - Jakub Zeman
- a Laboratory of Regulation of Gene Expression , Institute of Microbiology ASCR , Videnska, Prague , the Czech Republic
| | - Kristýna Poncová
- a Laboratory of Regulation of Gene Expression , Institute of Microbiology ASCR , Videnska, Prague , the Czech Republic
| | - Leoš Shivaya Valášek
- a Laboratory of Regulation of Gene Expression , Institute of Microbiology ASCR , Videnska, Prague , the Czech Republic
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10
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Antony A C, Alone PV. Defect in the GTPase activating protein (GAP) function of eIF5 causes repression of GCN4 translation. Biochem Biophys Res Commun 2017; 486:1110-1115. [PMID: 28385532 DOI: 10.1016/j.bbrc.2017.04.002] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/21/2017] [Accepted: 04/01/2017] [Indexed: 10/19/2022]
Abstract
In eukaryotes, the eIF5 protein plays an important role in translation start site selection by providing the GAP (GTPase activating protein) function. However, in yeast translation initiation fidelity defective eIF5G31R mutant causes preferential utilization of UUG as initiation codon and is termed as Suppressor of initiation codon (Sui-) phenotype due to its hyper GTPase activity. The eIF5G31R mutant dominantly represses GCN4 expression and confers sensitivity to 3-Amino-1,2,4-Trizole (3AT) induced starvation. The down-regulation of the GCN4 expression (Gcn- phenotype) in the eIF5G31R mutant was not because of leaky scanning defects; rather was due to the utilization of upUUG initiation codons at the 5' regulatory region present between uORF1 and the main GCN4 ORF.
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Affiliation(s)
- Charles Antony A
- School of Biological Sciences, National Institute of Science Education and Research Bhubaneswar, Constituent Institutes of Homi Bhabha National Institute (HBNI), P.O Jatni, Khurda 752050 India
| | - Pankaj V Alone
- School of Biological Sciences, National Institute of Science Education and Research Bhubaneswar, Constituent Institutes of Homi Bhabha National Institute (HBNI), P.O Jatni, Khurda 752050 India.
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11
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Mechanism and Regulation of Protein Synthesis in Saccharomyces cerevisiae. Genetics 2017; 203:65-107. [PMID: 27183566 DOI: 10.1534/genetics.115.186221] [Citation(s) in RCA: 93] [Impact Index Per Article: 13.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/17/2015] [Accepted: 02/24/2016] [Indexed: 12/18/2022] Open
Abstract
In this review, we provide an overview of protein synthesis in the yeast Saccharomyces cerevisiae The mechanism of protein synthesis is well conserved between yeast and other eukaryotes, and molecular genetic studies in budding yeast have provided critical insights into the fundamental process of translation as well as its regulation. The review focuses on the initiation and elongation phases of protein synthesis with descriptions of the roles of translation initiation and elongation factors that assist the ribosome in binding the messenger RNA (mRNA), selecting the start codon, and synthesizing the polypeptide. We also examine mechanisms of translational control highlighting the mRNA cap-binding proteins and the regulation of GCN4 and CPA1 mRNAs.
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12
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Young SK, Wek RC. Upstream Open Reading Frames Differentially Regulate Gene-specific Translation in the Integrated Stress Response. J Biol Chem 2016; 291:16927-35. [PMID: 27358398 DOI: 10.1074/jbc.r116.733899] [Citation(s) in RCA: 243] [Impact Index Per Article: 30.4] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Translation regulation largely occurs during initiation, which features ribosome assembly onto mRNAs and selection of the translation start site. Short, upstream ORFs (uORFs) located in the 5'-leader of the mRNA can be selected for translation. Multiple transcripts associated with stress amelioration are preferentially translated through uORF-mediated mechanisms during activation of the integrated stress response (ISR) in which phosphorylation of the α subunit of eIF2 results in a coincident global reduction in translation initiation. This review presents key features of uORFs that serve to optimize translational control that is essential for regulation of cell fate in response to environmental stresses.
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Affiliation(s)
- Sara K Young
- From the Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Indianapolis, Indiana 46202-5126
| | - Ronald C Wek
- From the Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Indianapolis, Indiana 46202-5126
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13
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Gunišová S, Beznosková P, Mohammad MP, Vlčková V, Valášek LS. In-depth analysis of cis-determinants that either promote or inhibit reinitiation on GCN4 mRNA after translation of its four short uORFs. RNA (NEW YORK, N.Y.) 2016; 22:542-558. [PMID: 26822200 PMCID: PMC4793210 DOI: 10.1261/rna.055046.115] [Citation(s) in RCA: 30] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/26/2015] [Accepted: 12/18/2015] [Indexed: 05/29/2023]
Abstract
Translational control in eukaryotes is exerted by many means, one of which involves a ribosome translating multiple cistrons per mRNA as in bacteria. It is called reinitiation (REI) and occurs on mRNAs where the main ORF is preceded by a short upstream uORF(s). Some uORFs support efficient REI on downstream cistrons, whereas some others do not. The mRNA of yeast transcriptional activator GCN4 contains four uORFs of both types that together compose an intriguing regulatory mechanism of its expression responding to nutrients' availability and various stresses. Here we subjected all GCN4 uORFs to a comprehensive analysis to identify all REI-promoting and inhibiting cis-determinants that contribute either autonomously or in synergy to the overall efficiency of REI on GCN4. We found that the 3' sequences of uORFs 1-3 contain a conserved AU1-2A/UUAU2 motif that promotes REI in position-specific, autonomous fashion such as the REI-promoting elements occurring in 5' sequences of uORF1 and uORF2. We also identified autonomous and transferable REI-inhibiting elements in the 3' sequences of uORF2 and uORF3, immediately following their AU-rich motif. Furthermore, we analyzed contributions of coding triplets and terminating stop codon tetranucleotides of GCN4 uORFs showing a negative correlation between the efficiency of reinitiation and efficiency of translation termination. Together we provide a complex overview of all cis-determinants of REI with their effects set in the context of the overall GCN4 translational control.
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Affiliation(s)
- Stanislava Gunišová
- Laboratory of Regulation of Gene Expression, Institute of Microbiology AS CR, Prague 142 20, Czech Republic
| | - Petra Beznosková
- Laboratory of Regulation of Gene Expression, Institute of Microbiology AS CR, Prague 142 20, Czech Republic
| | - Mahabub Pasha Mohammad
- Laboratory of Regulation of Gene Expression, Institute of Microbiology AS CR, Prague 142 20, Czech Republic
| | - Vladislava Vlčková
- Laboratory of Regulation of Gene Expression, Institute of Microbiology AS CR, Prague 142 20, Czech Republic
| | - Leoš Shivaya Valášek
- Laboratory of Regulation of Gene Expression, Institute of Microbiology AS CR, Prague 142 20, Czech Republic
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14
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Manickam N, Joshi K, Bhatt MJ, Farabaugh PJ. Effects of tRNA modification on translational accuracy depend on intrinsic codon-anticodon strength. Nucleic Acids Res 2015; 44:1871-81. [PMID: 26704976 PMCID: PMC4770228 DOI: 10.1093/nar/gkv1506] [Citation(s) in RCA: 74] [Impact Index Per Article: 8.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/03/2015] [Accepted: 12/14/2015] [Indexed: 12/14/2022] Open
Abstract
Cellular health and growth requires protein synthesis to be both efficient to ensure sufficient production, and accurate to avoid producing defective or unstable proteins. The background of misreading error frequency by individual tRNAs is as low as 2 × 10−6 per codon but is codon-specific with some error frequencies above 10−3 per codon. Here we test the effect on error frequency of blocking post-transcriptional modifications of the anticodon loops of four tRNAs in Escherichia coli. We find two types of responses to removing modification. Blocking modification of \documentclass[12pt]{minimal}
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}{}${\rm tRNA}_{{\rm UUC}}^{{\rm Glu}}$\end{document} and \documentclass[12pt]{minimal}
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}{}${\rm tRNA}^{\rm Asp}_{\rm QUC}$\end{document} increases errors, suggesting that the modifications act at least in part to maintain accuracy. Blocking even identical modifications of \documentclass[12pt]{minimal}
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}{}${\rm tRNA}^{\rm Lys}_{\rm UUU}$\end{document} and \documentclass[12pt]{minimal}
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}{}${\rm tRNA}^{\rm Tyr}_{\rm QUA}$\end{document} has the opposite effect of decreasing errors. One explanation could be that the modifications play opposite roles in modulating misreading by the two classes of tRNAs. Given available evidence that modifications help preorder the anticodon to allow it to recognize the codons, however, the simpler explanation is that unmodified ‘weak’ tRNAs decode too inefficiently to compete against cognate tRNAs that normally decode target codons, which would reduce the frequency of misreading.
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Affiliation(s)
- Nandini Manickam
- Department of Biological Sciences and Program in Molecular and Cell Biology, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
| | - Kartikeya Joshi
- Department of Biological Sciences and Program in Molecular and Cell Biology, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
| | - Monika J Bhatt
- Department of Biological Sciences and Program in Molecular and Cell Biology, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
| | - Philip J Farabaugh
- Department of Biological Sciences and Program in Molecular and Cell Biology, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
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15
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Wethmar K. The regulatory potential of upstream open reading frames in eukaryotic gene expression. WILEY INTERDISCIPLINARY REVIEWS-RNA 2014; 5:765-78. [DOI: 10.1002/wrna.1245] [Citation(s) in RCA: 127] [Impact Index Per Article: 12.7] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/21/2014] [Revised: 05/09/2014] [Accepted: 05/09/2014] [Indexed: 01/04/2023]
Affiliation(s)
- Klaus Wethmar
- Max-Delbrueck-Center for Molecular Medicine; Berlin Germany
- Helios Klinikum Berlin-Buch; Berlin Germany
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16
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Schiller C, Nowak C, Diakopoulos KN, Weidle UH, Weiss EH. An upstream open reading frame regulates LST1 expression during monocyte differentiation. PLoS One 2014; 9:e96245. [PMID: 24816991 PMCID: PMC4015914 DOI: 10.1371/journal.pone.0096245] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/22/2013] [Accepted: 04/04/2014] [Indexed: 01/23/2023] Open
Abstract
The regulation of gene expression depends on the interplay of multiple factors at the transcriptional and translational level. Upstream open reading frames (uORFs) play an important role as translational repressors of main ORFs and their presence or usage in transcripts can be regulated by different mechanisms. The main objective of the present study was to assess whether uORFs regulate the expression of the MHC class III gene LST1. We report that expression of LST1 is tightly regulated by alternative transcription initiation and the presence of an uORF in the 5′-UTR of transcripts. Specifically, using EGFP reporter constructs in human HeLa and HEK-293T cells and flow cytometry as well as western blot analysis we found the uORF to reduce the expression of the main ORF by roughly two-thirds. Furthermore, we were able to correlate a previously detected increase in LST1 protein expression during monocyte differentiation with an increase of transcription initiation at an alternative exon that does not contain an uORF.
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Affiliation(s)
- Christian Schiller
- Department of Biology II, Ludwig-Maximilians-Universität München, Germany
| | - Carina Nowak
- Department of Biology II, Ludwig-Maximilians-Universität München, Germany
| | | | | | - Elisabeth H. Weiss
- Department of Biology II, Ludwig-Maximilians-Universität München, Germany
- * E-mail:
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17
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Nonsense-mediated mRNA decay immunity can help identify human polycistronic transcripts. PLoS One 2014; 9:e91535. [PMID: 24621851 PMCID: PMC3951408 DOI: 10.1371/journal.pone.0091535] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/21/2012] [Accepted: 02/13/2014] [Indexed: 11/19/2022] Open
Abstract
Eukaryotic polycistronic transcription units are rare and only a few examples are known, mostly being the outcome of serendipitous discovery. We claim that nonsense-mediated mRNA decay (NMD) immune structure is a common characteristic of polycistronic transcripts, and that this immunity is an emergent property derived from all functional CDSs. The human RefSeq transcriptome was computationally screened for transcripts capable of eliciting NMD, and which contain an additional ORF(s) potentially capable of rescuing the transcript from NMD. Transcripts were further analyzed implementing domain-based strategies in order to estimate the potential of the candidate ORF to encode a functional protein. Consequently, we predict the existence of forty nine novel polycistronic transcripts. Experimental verification was carried out utilizing two different types of analyses. First, five Gene Expression Omnibus (GEO) datasets from published NMD-inhibition studies were used, aiming to explore whether a given mRNA is indeed insensitive to NMD. All known bicistronic transcripts and eleven out of the twelve predicted genes that were analyzed, displayed NMD insensitivity using various NMD inhibitors. For three genes, a mixed expression pattern was observed presenting both NMD sensitivity and insensitivity in different cell types. Second, we used published global translation initiation sequencing data from HEK293 cells to verify the existence of translation initiation sites in our predicted polycistronic genes. In five of our genes, the predicted rescuing uORFs are indeed identified as translation initiation sites, and in two additional genes, one of two predicted rescuing uORF is verified. These results validate our computational analysis and reinforce the possibility that NMD-immune architecture is a parameter by which polycistronic genes can be identified. Moreover, we present evidence for NMD-mediated regulation controlling the production of one or more proteins encoded in the polycistronic transcript.
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18
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Gunišová S, Valášek LS. Fail-safe mechanism of GCN4 translational control--uORF2 promotes reinitiation by analogous mechanism to uORF1 and thus secures its key role in GCN4 expression. Nucleic Acids Res 2014; 42:5880-93. [PMID: 24623812 PMCID: PMC4027193 DOI: 10.1093/nar/gku204] [Citation(s) in RCA: 35] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023] Open
Abstract
One of the extensively studied mechanisms of gene-specific translational regulation is reinitiation. It takes place on messenger RNAs (mRNAs) where main ORF is preceded by upstream ORF (uORF). Even though uORFs generally down-regulate main ORF expression, specific uORFs exist that allow high level of downstream ORF expression. The key is their ability to retain 40S subunits on mRNA upon termination of their translation to resume scanning for the next AUG. Here, we took advantage of the exemplary model system of reinitiation, the mRNA of yeast transcriptional activator GCN4 containing four short uORFs, and show that contrary to previous reports, not only the first but the first two of its uORFs allow efficient reinitiation. Strikingly, we demonstrate that they utilize a similar molecular mechanism relying on several cis-acting 5' reinitiation-promoting elements, one of which they share, and the interaction with the a/TIF32 subunit of translation initiation factor eIF3. Since a similar mechanism operates also on YAP1 uORF, our findings strongly suggest that basic principles of reinitiation are conserved. Furthermore, presence of two consecutive reinitiation-permissive uORFs followed by two reinitiation-non-permissive uORFs suggests that tightness of GCN4 translational control is ensured by a fail-safe mechanism that effectively prevents or triggers GCN4 expression under nutrient replete or deplete conditions, respectively.
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Affiliation(s)
- Stanislava Gunišová
- Laboratory of Regulation of Gene Expression, Institute of Microbiology ASCR, Videnska 1083, 142 20 Prague, the Czech Republic
| | - Leoš Shivaya Valášek
- Laboratory of Regulation of Gene Expression, Institute of Microbiology ASCR, Videnska 1083, 142 20 Prague, the Czech Republic
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19
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Translation reinitiation relies on the interaction between eIF3a/TIF32 and progressively folded cis-acting mRNA elements preceding short uORFs. PLoS Genet 2011; 7:e1002137. [PMID: 21750682 PMCID: PMC3131280 DOI: 10.1371/journal.pgen.1002137] [Citation(s) in RCA: 68] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/22/2011] [Accepted: 05/03/2011] [Indexed: 11/19/2022] Open
Abstract
Reinitiation is a gene-specific translational control mechanism characterized by the ability of some short upstream uORFs to retain post-termination 40S subunits on mRNA. Its efficiency depends on surrounding cis-acting sequences, uORF elongation rates, various initiation factors, and the intercistronic distance. To unravel effects of cis-acting sequences, we investigated previously unconsidered structural properties of one such a cis-enhancer in the mRNA leader of GCN4 using yeast genetics and biochemistry. This leader contains four uORFs but only uORF1, flanked by two transferrable 5' and 3' cis-acting sequences, and allows efficient reinitiation. Recently we showed that the 5' cis-acting sequences stimulate reinitiation by interacting with the N-terminal domain (NTD) of the eIF3a/TIF32 subunit of the initiation factor eIF3 to stabilize post-termination 40S subunits on uORF1 to resume scanning downstream. Here we identify four discernible reinitiation-promoting elements (RPEs) within the 5' sequences making up the 5' enhancer. Genetic epistasis experiments revealed that two of these RPEs operate in the eIF3a/TIF32-dependent manner. Likewise, two separate regions in the eIF3a/TIF32-NTD were identified that stimulate reinitiation in concert with the 5' enhancer. Computational modeling supported by experimental data suggests that, in order to act, the 5' enhancer must progressively fold into a specific secondary structure while the ribosome scans through it prior uORF1 translation. Finally, we demonstrate that the 5' enhancer's stimulatory activity is strictly dependent on and thus follows the 3' enhancer's activity. These findings allow us to propose for the first time a model of events required for efficient post-termination resumption of scanning. Strikingly, structurally similar RPE was predicted and identified also in the 5' leader of reinitiation-permissive uORF of yeast YAP1. The fact that it likewise operates in the eIF3a/TIF32-dependent manner strongly suggests that at least in yeasts the underlying mechanism of reinitiation on short uORFs is conserved.
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20
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Cuchalová L, Kouba T, Herrmannová A, Dányi I, Chiu WL, Valášek L. The RNA recognition motif of eukaryotic translation initiation factor 3g (eIF3g) is required for resumption of scanning of posttermination ribosomes for reinitiation on GCN4 and together with eIF3i stimulates linear scanning. Mol Cell Biol 2010; 30:4671-86. [PMID: 20679478 PMCID: PMC2950517 DOI: 10.1128/mcb.00430-10] [Citation(s) in RCA: 85] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/13/2010] [Revised: 05/25/2010] [Accepted: 07/26/2010] [Indexed: 12/15/2022] Open
Abstract
Recent reports have begun unraveling the details of various roles of individual eukaryotic translation initiation factor 3 (eIF3) subunits in translation initiation. Here we describe functional characterization of two essential Saccharomyces cerevisiae eIF3 subunits, g/Tif35 and i/Tif34, previously suggested to be dispensable for formation of the 48S preinitiation complexes (PICs) in vitro. A triple-Ala substitution of conserved residues in the RRM of g/Tif35 (g/tif35-KLF) or a single-point mutation in the WD40 repeat 6 of i/Tif34 (i/tif34-Q258R) produces severe growth defects and decreases the rate of translation initiation in vivo without affecting the integrity of eIF3 and formation of the 43S PICs in vivo. Both mutations also diminish induction of GCN4 expression, which occurs upon starvation via reinitiation. Whereas g/tif35-KLF impedes resumption of scanning for downstream reinitiation by 40S ribosomes terminating at upstream open reading frame 1 (uORF1) in the GCN4 mRNA leader, i/tif34-Q258R prevents full GCN4 derepression by impairing the rate of scanning of posttermination 40S ribosomes moving downstream from uORF1. In addition, g/tif35-KLF reduces processivity of scanning through stable secondary structures, and g/Tif35 specifically interacts with Rps3 and Rps20 located near the ribosomal mRNA entry channel. Together these results implicate g/Tif35 and i/Tif34 in stimulation of linear scanning and, specifically in the case of g/Tif35, also in proper regulation of the GCN4 reinitiation mechanism.
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Affiliation(s)
- Lucie Cuchalová
- Laboratory of Regulation of Gene Expression, Institute of Microbiology AVCR, v.v.i., Prague, Czech Republic, Laboratory of Gene Regulation and Development, Eunice Kennedy Shriver National Institute of Child Health and Human Development, NIH, Bethesda, Maryland 20892
| | - Tomáš Kouba
- Laboratory of Regulation of Gene Expression, Institute of Microbiology AVCR, v.v.i., Prague, Czech Republic, Laboratory of Gene Regulation and Development, Eunice Kennedy Shriver National Institute of Child Health and Human Development, NIH, Bethesda, Maryland 20892
| | - Anna Herrmannová
- Laboratory of Regulation of Gene Expression, Institute of Microbiology AVCR, v.v.i., Prague, Czech Republic, Laboratory of Gene Regulation and Development, Eunice Kennedy Shriver National Institute of Child Health and Human Development, NIH, Bethesda, Maryland 20892
| | - István Dányi
- Laboratory of Regulation of Gene Expression, Institute of Microbiology AVCR, v.v.i., Prague, Czech Republic, Laboratory of Gene Regulation and Development, Eunice Kennedy Shriver National Institute of Child Health and Human Development, NIH, Bethesda, Maryland 20892
| | - Wen-ling Chiu
- Laboratory of Regulation of Gene Expression, Institute of Microbiology AVCR, v.v.i., Prague, Czech Republic, Laboratory of Gene Regulation and Development, Eunice Kennedy Shriver National Institute of Child Health and Human Development, NIH, Bethesda, Maryland 20892
| | - Leoš Valášek
- Laboratory of Regulation of Gene Expression, Institute of Microbiology AVCR, v.v.i., Prague, Czech Republic, Laboratory of Gene Regulation and Development, Eunice Kennedy Shriver National Institute of Child Health and Human Development, NIH, Bethesda, Maryland 20892
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21
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Szamecz B, Rutkai E, Cuchalová L, Munzarová V, Herrmannová A, Nielsen KH, Burela L, Hinnebusch AG, Valásek L. eIF3a cooperates with sequences 5' of uORF1 to promote resumption of scanning by post-termination ribosomes for reinitiation on GCN4 mRNA. Genes Dev 2008; 22:2414-25. [PMID: 18765792 DOI: 10.1101/gad.480508] [Citation(s) in RCA: 104] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
Abstract
Yeast initiation factor eIF3 (eukaryotic initiation factor 3) has been implicated in multiple steps of translation initiation. Previously, we showed that the N-terminal domain (NTD) of eIF3a interacts with the small ribosomal protein RPS0A located near the mRNA exit channel, where eIF3 is proposed to reside. Here, we demonstrate that a partial deletion of the RPS0A-binding domain of eIF3a impairs translation initiation and reduces binding of eIF3 and associated eIFs to native preinitiation complexes in vivo. Strikingly, it also severely blocks the induction of GCN4 translation that occurs via reinitiation. Detailed examination unveiled a novel reinitiation defect resulting from an inability of 40S ribosomes to resume scanning after terminating at the first upstream ORF (uORF1). Genetic analysis reveals a functional interaction between the eIF3a-NTD and sequences 5' of uORF1 that is critically required to enhance reinitiation. We further demonstrate that these stimulatory sequences must be positioned precisely relative to the uORF1 stop codon and that reinitiation efficiency after uORF1 declines with its increasing length. Together, our results suggest that eIF3 is retained on ribosomes throughout uORF1 translation and, upon termination, interacts with its 5' enhancer at the mRNA exit channel to stabilize mRNA association with post-termination 40S subunits and enable resumption of scanning for reinitiation downstream.
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Affiliation(s)
- Béla Szamecz
- Laboratory of Regulation of Gene Expression, Institute of Microbiology AVCR, Prague, Videnska 1083, 142 20, The Czech Republic
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22
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Abstract
Cells reprogram gene expression in response to environmental changes by mobilizing transcriptional activators. The activator protein Gcn4 of the yeast Saccharomyces cerevisiae is regulated by an intricate translational control mechanism, which is the primary focus of this review, and also by the modulation of its stability in response to nutrient availability. Translation of GCN4 mRNA is derepressed in amino acid-deprived cells, leading to transcriptional induction of nearly all genes encoding amino acid biosynthetic enzymes. The trans-acting proteins that control GCN4 translation have general functions in the initiation of protein synthesis, or regulate the activities of initiation factors, so that the molecular events that induce GCN4 translation also reduce the rate of general protein synthesis. This dual regulatory response enables cells to limit their consumption of amino acids while diverting resources into amino acid biosynthesis in nutrient-poor environments. Remarkably, mammalian cells use the same strategy to downregulate protein synthesis while inducing transcriptional activators of stress-response genes under various stressful conditions, including amino acid starvation.
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Affiliation(s)
- Alan G Hinnebusch
- Laboratory of Gene Regulation and Development, National Institute of Child Health and Human Development, Bethesda, Maryland 20892, USA.
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23
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Vattem KM, Wek RC. Reinitiation involving upstream ORFs regulates ATF4 mRNA translation in mammalian cells. Proc Natl Acad Sci U S A 2004; 101:11269-74. [PMID: 15277680 PMCID: PMC509193 DOI: 10.1073/pnas.0400541101] [Citation(s) in RCA: 1216] [Impact Index Per Article: 60.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
During cellular stresses, phosphorylation of eukaryotic initiation factor-2 (eIF2) elicits gene expression designed to ameliorate the underlying cellular disturbance. Central to this stress response is the transcriptional regulator activating transcription factor, ATF4. Here we describe the mechanism regulating ATF4 expression involving the differential contribution of two upstream ORFs (uORFs) in the 5' leader of the mouse ATF4 mRNA. The 5' proximal uORF1 is a positive-acting element that facilitates ribosome scanning and reinitiation at downstream coding regions in the ATF4 mRNA. When eIF2-GTP is abundant in nonstressed cells, ribosomes scanning downstream of uORF1 reinitiate at the next coding region, uORF2, an inhibitory element that blocks ATF4 expression. During stress conditions, phosphorylation of eIF2 and the accompanying reduction in the levels of eIF2-GTP increase the time required for the scanning ribosomes to become competent to reinitiate translation. This delayed reinitiation allows for ribosomes to scan through the inhibitory uORF2 and instead reinitiate at the ATF4-coding region. Increased expression of ATF4 would contribute to the expression of genes involved in remediation of cellular stress damage. These results suggest that the mechanism of translation reinitiation involving uORFs is conserved from yeast to mammals.
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Affiliation(s)
- Krishna M Vattem
- Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Indianapolis, IN 46202, USA
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24
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Zhang P, McGrath BC, Reinert J, Olsen DS, Lei L, Gill S, Wek SA, Vattem KM, Wek RC, Kimball SR, Jefferson LS, Cavener DR. The GCN2 eIF2alpha kinase is required for adaptation to amino acid deprivation in mice. Mol Cell Biol 2002; 22:6681-8. [PMID: 12215525 PMCID: PMC134046 DOI: 10.1128/mcb.22.19.6681-6688.2002] [Citation(s) in RCA: 343] [Impact Index Per Article: 15.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022] Open
Abstract
The GCN2 eIF2alpha kinase is essential for activation of the general amino acid control pathway in yeast when one or more amino acids become limiting for growth. GCN2's function in mammals is unknown, but must differ, since mammals, unlike yeast, can synthesize only half of the standard 20 amino acids. To investigate the function of mammalian GCN2, we have generated a Gcn2(-/-) knockout strain of mice. Gcn2(-/-) mice are viable, fertile, and exhibit no phenotypic abnormalities under standard growth conditions. However, prenatal and neonatal mortalities are significantly increased in Gcn2(-/-) mice whose mothers were reared on leucine-, tryptophan-, or glycine-deficient diets during gestation. Leucine deprivation produced the most pronounced effect, with a 63% reduction in the expected number of viable neonatal mice. Cultured embryonic stem cells derived from Gcn2(-/-) mice failed to show the normal induction of eIF2alpha phosphorylation in cells deprived of leucine. To assess the biochemical effects of the loss of GCN2 in the whole animal, liver perfusion experiments were conducted. Histidine limitation in the presence of histidinol induced a twofold increase in the phosphorylation of eIF2alpha and a concomitant reduction in eIF2B activity in perfused livers from wild-type mice, but no changes in livers from Gcn2(-/-) mice.
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Affiliation(s)
- Peichuan Zhang
- Department of Biology, The Pennsylvania State University, University Park 16802, USA
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25
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Meijer HA, Thomas AAM. Control of eukaryotic protein synthesis by upstream open reading frames in the 5'-untranslated region of an mRNA. Biochem J 2002; 367:1-11. [PMID: 12117416 PMCID: PMC1222879 DOI: 10.1042/bj20011706] [Citation(s) in RCA: 233] [Impact Index Per Article: 10.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/22/2001] [Revised: 06/25/2002] [Accepted: 07/15/2002] [Indexed: 11/17/2022]
Abstract
Control of gene expression is achieved at various levels. Translational control becomes crucial in the absence of transcription, such as occurs in early developmental stages. One of the initiating events in translation is that the 40 S subunit of the ribosome binds the mRNA at the 5'-cap structure and scans the 5'-untranslated region (5'-UTR) for AUG initiation codons. AUG codons upstream of the main open reading frame can induce formation of a translation-competent ribosome that may translate and (i) terminate and re-initiate, (ii) terminate and leave the mRNA, resulting in down-regulation of translation of the main open reading frame, or (iii) synthesize an N-terminally extended protein. In the present review we discuss how upstream AUGs can control the expression of the main open reading frame, and a comparison is made with other elements in the 5'-UTR that control mRNA translation, such as hairpins and internal ribosome entry sites. Recent data indicate the flexibility of controlling translation initiation, and how the mode of ribosome entry on the mRNA as well as the elements in the 5'-UTR can accurately regulate the amount of protein synthesized from a specific mRNA.
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Affiliation(s)
- Hedda A Meijer
- Department of Developmental Biology, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands.
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Kimball SR. Regulation of translation initiation by amino acids in eukaryotic cells. PROGRESS IN MOLECULAR AND SUBCELLULAR BIOLOGY 2002; 26:155-84. [PMID: 11575165 DOI: 10.1007/978-3-642-56688-2_6] [Citation(s) in RCA: 77] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
Abstract
The translation of mRNA in eukaryotic cells is regulated by amino acids through multiple mechanisms. One such mechanism involves activation of mTOR (Fig. 1). mTOR controls a myriad of downstream effectors, including RNA polymerase I, S6K1, 4E-BP1, and eEF2 kinase. In yeast, and probably in higher eukaryotes, mTOR signals through Tap42p/alpha 4 to regulate protein phosphatases. Through phosphorylation of Tap42p/alpha 4, mTOR abrogates dephosphorylation of the downstream effectors by PP2 A and/or PP6, resulting in their increased phosphorylation. Although at this time still speculative, in vitro results using mTOR immunoprecipitates suggest that mTOR, or an associated kinase, may also be directly involved in phosphorylating some effectors. Enhanced RNA polymerase I activity results in increased transcription of rDNA genes, whereas increased S6K1 activity promotes preferential translation of TOP mRNAs, such as those encoding ribosomal proteins. Together, stimulated RNA polymerase I and S6K1 activities enhance ribosome biogenesis, increasing the translational capacity of the cell. Phosphorylation of 4E-BP1 prohibits its association with eIF4E, allowing eIF4E to bind to eIF4G and form the active eIF4F complex. Increased eIF4F formation preferentially stimulates translation of mRNAs containing long, highly-structured 5' UTRs. Finally, amino acids cause inhibition of the eEF2 kinase, resulting in an increase in the proportion of eEF2 in the active, dephosphorylated form. By inhibiting eEF2 phosphorylation, amino acids may not only stimulate translation elongation, but may also prevent activation of GCN2 by enhancing the rate of removal of deacylated tRNA from the P-site on the ribosome; a potential activator of GCN2. GCN2 may also be regulated directly by the accumulation of deacylated-tRNA caused by treatment with inhibitors of tRNA synthetases or in cells incubated in the absence of essential amino acids. However, because the Km of the tRNA synthetases for amino acids is well above the amino acid concentrations found in plasma of fasted animals, such a mechanism may not be operative in mammals in vivo. Activation of GCN2 results in increased phosphorylation of the alpha-subunit of eIF2, which in turn causes inhibition of eIF2B. Thus, by preventing activation of GCN2, amino acids preserve eIF2B activity, which promotes translation of all mRNAs, i.e., global protein synthesis is enhanced.
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MESH Headings
- Adaptor Proteins, Signal Transducing
- Amino Acids, Essential/metabolism
- Animals
- Carrier Proteins/metabolism
- Cell Cycle Proteins
- DNA-Binding Proteins
- Eukaryotic Initiation Factor-2/metabolism
- Eukaryotic Initiation Factor-2B/metabolism
- Fungal Proteins/genetics
- Humans
- Models, Biological
- Peptide Chain Initiation, Translational/physiology
- Phosphoproteins/metabolism
- Phosphorylation
- Protein Kinases/genetics
- Protein Kinases/metabolism
- Protein Serine-Threonine Kinases
- RNA, Fungal/genetics
- RNA, Fungal/metabolism
- RNA, Messenger/genetics
- RNA, Messenger/metabolism
- RNA, Transfer, Met/metabolism
- Ribosomal Protein S6 Kinases/metabolism
- Ribosomes/metabolism
- Saccharomyces cerevisiae/metabolism
- Saccharomyces cerevisiae Proteins
- Signal Transduction
- eIF-2 Kinase/metabolism
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Affiliation(s)
- S R Kimball
- Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033, USA
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27
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Ryabova LA, Pooggin MM, Hohn T. Viral strategies of translation initiation: ribosomal shunt and reinitiation. PROGRESS IN NUCLEIC ACID RESEARCH AND MOLECULAR BIOLOGY 2002; 72:1-39. [PMID: 12206450 PMCID: PMC7133299 DOI: 10.1016/s0079-6603(02)72066-7] [Citation(s) in RCA: 82] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
Due to the compactness of their genomes, viruses are well suited to the study of basic expression mechanisms, including details of transcription, RNA processing, transport, and translation. In fact, most basic principles of these processes were first described in viral systems. Furthermore, viruses seem not to respect basic rules, and cases of "abnormal" expression strategies are quiet common, although such strategies are usually also finally observed in rare cases of cellular gene expression. Concerning translation, viruses most often violate Kozak's original rule that eukaryotic translation starts from a capped monocistronic mRNA and involves linear scanning to find the first suitable start codon. Thus, many viral cases have been described where translation is initiated from noncapped RNA, using an internal ribosome entry site. This review centers on other viral translation strategies, namely shunting and virus-controlled reinitiation as first described in plant pararetroviruses (Caulimoviridae). In shunting, major parts of a complex leader are bypassed and not melted by scanning ribosomes. In the Caulimoviridae, this process is coupled to reinitiation after translation of a small open reading frame; in other cases, it is possibly initiated upon pausing of the scanning ribosome. Most of the Caulimoviridae produce polycistronic mRNAs. Two basic mechanisms are used for their translation. Alternative translation of the downstream open reading frames in the bacilliform Caulimoviridae occurs by a leaky scanning mechanism, and reinitiation of polycistronic translation in many of the icosahedral Caulimoviridae is enabled by the action of a viral transactivator. Both of these processes are discussed here in detail and compared to related processes in other viruses and cells.
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28
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Hoffmann B, Valerius O, Andermann M, Braus GH. Transcriptional autoregulation and inhibition of mRNA translation of amino acid regulator gene cpcA of filamentous fungus Aspergillus nidulans. Mol Biol Cell 2001; 12:2846-57. [PMID: 11553722 PMCID: PMC59718 DOI: 10.1091/mbc.12.9.2846] [Citation(s) in RCA: 68] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
The CPCA protein of the filamentous fungus Aspergillus nidulans is a member of the c-Jun-like transcriptional activator family. It acts as central transcription factor of the cross-pathway regulatory network of amino acid biosynthesis and is functionally exchangeable for the general control transcriptional activator Gcn4p of Saccharomyces cerevisiae. In contrast to GCN4, expression of cpcA is strongly regulated by two equally important mechanisms with additive effects that lead to a fivefold increased CPCA protein amount under amino acid starvation conditions. One component of cpcA regulation involves a transcriptional autoregulatory mechanism via a CPCA recognition element (CPRE) in the cpcA promoter that causes a sevenfold increased cpcA mRNA level when cells are starved for amino acids. Point mutations in the CPRE cause a constitutively low mRNA level of cpcA and a halved protein level when amino acids are limited. Moreover, two upstream open reading frames (uORFs) in the 5' region of the cpcA mRNA are important for a translational regulatory mechanism. Destruction of both short uORFs results in a sixfold increased CPCA protein level under nonstarvation conditions and a 10-fold increase under starvation conditions. Mutations in both the CPRE and uORF regulatory elements lead to an intermediate effect, with a low cpcA mRNA level but a threefold increased CPCA protein level independent of amino acid availability. These data argue for a combined regulation of cpcA that includes a translational regulation like that of yeast GCN4 as well as a transcriptional regulation like that of the mammalian jun and fos genes.
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Affiliation(s)
- B Hoffmann
- Institute of Microbiology and Genetics, Georg-August University, D-37077 Göttingen, Germany
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29
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Hemmings-Mieszczak M, Hohn T, Preiss T. Termination and peptide release at the upstream open reading frame are required for downstream translation on synthetic shunt-competent mRNA leaders. Mol Cell Biol 2000; 20:6212-23. [PMID: 10938098 PMCID: PMC86096 DOI: 10.1128/mcb.20.17.6212-6223.2000] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
We have shown recently that a stable hairpin preceded by a short upstream open reading frame (uORF) promotes nonlinear ribosome migration or ribosome shunt on a synthetic mRNA leader (M. Hemmings-Mieszczak and T. Hohn, RNA 5:1149-1157, 1999). We have now used the model mRNA leader to study further the mechanism of shunting in vivo and in vitro. We show that a full cycle of translation of the uORF, including initiation, elongation, and termination, is a precondition for the ribosome shunt across the stem structure to initiate translation downstream. Specifically, AUG recognition and the proper release of the nascent peptide are necessary and sufficient for shunting. Furthermore, the stop codon context must not impede downstream reinitiation. Translation of the main ORF was inhibited by replacement of the uORF by coding sequences repressing reinitiation but stimulated by the presence of the virus-specific translational transactivator of reinitiation (cauliflower mosaic virus pVI). Our results indicate reinitiation as the mechanism of translation initiation on the synthetic shunt-competent mRNA leader and suggest that uORF-dependent shunting is more prevalent than previously anticipated. Within the above constraints, uORF-dependent shunting is quite tolerant of uORF and stem sequences and operates in systems as diverse as plants and fungi.
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30
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Ryabova LA, Hohn T. Ribosome shunting in the cauliflower mosaic virus 35S RNA leader is a special case of reinitiation of translation functioning in plant and animal systems. Genes Dev 2000. [DOI: 10.1101/gad.14.7.817] [Citation(s) in RCA: 38] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/11/2023]
Abstract
The shunt model predicts that small ORFs (sORFs) within the cauliflower mosaic virus (CaMV) 35S RNA leader and downstream ORF VII are translated by different mechanisms, that is, scanning–reinitiation and shunting, respectively. Wheat germ extract (WGE) and rabbit reticulocyte lysate (RRL) in vitro translation systems were used to discriminate between these two processes and to study the mechanism of ribosomal shunt. In both systems, expression downstream of the leader occurred via ribosomal shunt under the control of a stable stem and a small ORF preceding it. Shunting ribosomes were also able to initiate quite efficiently at non-AUG start codons just downstream of the shunt landing site in WGE but not in RRL. The short sORF MAGDIS from the mammalian AdoMetDC RNA, which conditionally suppresses reinitiation at a downstream ORF, prevented shunting if placed at the position of sORF A, the 5′-proximal ORF of the CaMV leader. We have demonstrated directly that sORF A is translated and that proper termination of translation at the 5′-proximal ORF is absolutely required for both shunting and linear ribosome migration. These findings strongly indicate that shunting is a special case of reinitiation.
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31
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Abstract
A conserved mRNA degradation system, referred to as mRNA surveillance, exists in eukaryotic cells to degrade aberrant mRNAs. A defining aspect of aberrant transcripts is that the spatial relationship between the termination codon and specific downstream sequence information has been altered. A key, yet unknown, feature of the mRNA surveillance system is how this spatial relationship is assessed in individual transcripts. Two views have emerged to describe how discrimination between proper and improper termination might occur. In the first view, a surveillance complex assembles onto the mRNA after translation termination, and scans the mRNA in a 3' to 5' direction for a limited distance. If specific downstream sequence information is encountered during this scanning, then the surveillance complex targets the transcript for rapid decay. An alternate view suggests that the downstream sequence information influences how translation termination occurs. This view encompasses several ideas including: (a) The architecture of the mRNP can alter the rate of key steps in translation termination; (b) the discrimination between a proper and improper termination occurs via an internal, Upf1-dependent, timing mechanism; and (c) proper termination results in the restructuring of the mRNP to a form that promotes mRNA stability. This proposed model for mRNA surveillance is similar to other systems of kinetic proofreading that monitor the accuracy of other biogenic processes such as translation and spliceosome assembly.
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Affiliation(s)
- P Hilleren
- Department of Molecular and Cellular Biology, Howard Hughes Medical Institute, University of Arizona, Tucson 85721, USA
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32
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González CI, Ruiz-Echevarría MJ, Vasudevan S, Henry MF, Peltz SW. The yeast hnRNP-like protein Hrp1/Nab4 marks a transcript for nonsense-mediated mRNA decay. Mol Cell 2000; 5:489-99. [PMID: 10882134 DOI: 10.1016/s1097-2765(00)80443-8] [Citation(s) in RCA: 117] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
Abstract
The nonsense-mediated mRNA decay (NMD) pathway monitors premature translation termination and degrades aberrant mRNAs. In yeast, it has been proposed that a surveillance complex searches 3' of a nonsense codon for a downstream sequence element (DSE) associated with RNA-binding proteins. An interaction between the complex and the DSE-binding protein(s) triggers NMD. Here we describe the identification and characterization of the Hrp1/Nab4 protein as a DSE-binding factor that activates NMD. Mutations in HRP1 stabilize nonsense-containing transcripts without affecting the decay of wild-type mRNAs. Hrp1p binds specifically to a DSE-containing RNA and interacts with Upf1p, a component of the surveillance complex. A mutation in HRP1 that stabilizes nonsense-containing mRNAs abolishes its affinity for the DSE and fails to interact with Upf1p. We present a model describing how Hrp1p marks a transcript for rapid decay.
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Affiliation(s)
- C I González
- Department of Molecular Genetics and Microbiology, Robert Wood Johnson Medical School, University of Medicine and Dentistry of New Jersey, Piscataway 08854, USA
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33
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Nakajima T, Cheng T, Rohrwasser A, Bloem LJ, Pratt JH, Inoue I, Lalouel JM. Functional analysis of a mutation occurring between the two in-frame AUG codons of human angiotensinogen. J Biol Chem 1999; 274:35749-55. [PMID: 10585456 DOI: 10.1074/jbc.274.50.35749] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Angiotensinogen (ANG) is the specific substrate of the renin-angiotensin system, a major participant in blood pressure control. We have identified a natural mutation at the -30 amino acid position of the angiotensinogen signal peptide, in which an arginine is replaced by a proline (R-30P). Heterozygous individuals with R-30P showed a tendency to lowered plasma angiotensinogen level (1563 ng of ANG I/ml (range 1129-1941)) compared with normal individuals in the family (1892 ng of ANG I/ml (range 1603-2072)). Human angiotensinogen mRNA has two in-phase translation initiation codons (AUG) starting upstream 39 and 66 nucleotides from the cap site. R-30P occurs in a cluster of basic residues adjacent to the first AUG codon that may affect intracellular sorting of the nascent protein. Pulse-chase experiments in transiently transfected cultured cells revealed that the R-30P mutation was associated with reduced amounts of both intra- and extracellular protein. In a cell-free system, we found that two forms of native angiotensinogen were generated by alternative initiation of translation at either AUG codon. Alteration of either the first or second AUG codons abolished the synthesis of the longer and the shorter form of native angiotensinogen, respectively. Furthermore, the rate of secretion of the shorter form was lower than that of the longer form. By transplanting angiotensinogen signal peptide onto green fluorescence protein, however, we found that both forms of the signal peptide could target green fluorescence protein, normally localized in the cytoplasm, to the secretory pathway. Although the R-30P mutation may not affect intracellular sorting of angiotensinogen in a qualitative manner, it leads to a quantitative reduction in the net secretion of mature angiotensinogen through decreased translocation or increased residence time in the endoplasmic reticulum.
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Affiliation(s)
- T Nakajima
- Howard Hughes Medical Institute, University of Utah Health Sciences Center, Salt Lake City, Utah 84112, USA
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34
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Abstract
Regulation of translation initiation is a central control point in animal cells. We review our current understanding of the mechanisms of regulation, drawing particularly on examples in which the biological consequences of the regulation are clear. Specific mRNAs can be controlled via sequences in their 5' and 3' untranslated regions (UTRs) and by alterations in the translation machinery. The 5'UTR sequence can determine which initiation pathway is used to bring the ribosome to the initiation codon, how efficiently initiation occurs, and which initiation site is selected. 5'UTR-mediated control can also be accomplished via sequence-specific mRNA-binding proteins. Sequences in the 3' untranslated region and the poly(A) tail can have dramatic effects on initiation frequency, with particularly profound effects in oogenesis and early development. The mechanism by which 3'UTRs and poly(A) regulate initiation may involve contacts between proteins bound to these regions and the basal translation apparatus. mRNA localization signals in the 3'UTR can also dramatically influence translational activation and repression. Modulations of the initiation machinery, including phosphorylation of initiation factors and their regulated association with other proteins, can regulate both specific mRNAs and overall translation rates and thereby affect cell growth and phenotype.
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Affiliation(s)
- N K Gray
- Department of Biochemistry, University of Wisconsin, Madison 53706, USA
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35
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Mize GJ, Ruan H, Low JJ, Morris DR. The inhibitory upstream open reading frame from mammalian S-adenosylmethionine decarboxylase mRNA has a strict sequence specificity in critical positions. J Biol Chem 1998; 273:32500-5. [PMID: 9829983 DOI: 10.1074/jbc.273.49.32500] [Citation(s) in RCA: 70] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The upstream open reading frame (uORF) in the 5' leader of the mammalian mRNA encoding S-adenosylmethionine decarboxylase (AdoMetDC) serves as a negative regulatory element by suppressing translation of the associated downstream cistron. Certain changes in the amino acid sequence of the hexapeptide (sequence MAGDIS) encoded by the uORF destroy suppressive activity, implying specific interaction with a cellular target. In this paper, we examine the extent of alterations that can be tolerated in this uORF. The mammalian AdoMetDC uORF inhibits downstream translation when placed into the 5' leader of a yeast mRNA with characteristics resembling those in mammalian cells, suggesting that the encoded peptide has a similar target across species. Using yeast for the initial screen, we tested the specificity of the critical three codons at the 3' end of the uORF by saturation mutagenesis. Altered uORFs selected from the primary yeast screen were then retested in mammalian cells. The requirements at codons 4 and 5 were quite stringent; only aspartic acid at codon 4 yielded a fully suppressive peptide, and only valine could substitute productively for isoleucine at codon 5. The specificity at codon 6 was much looser, with many substitutions retaining suppressive activity in both yeast and mammalian cells.
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Affiliation(s)
- G J Mize
- Department of Biochemistry, University of Washington, Seattle, Washington 98195-7350, USA
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36
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Mueller PP, Grueter P, Hinnebusch AG, Trachsel H. A ribosomal protein is required for translational regulation of GCN4 mRNA. Evidence for involvement of the ribosome in eIF2 recycling. J Biol Chem 1998; 273:32870-7. [PMID: 9830035 DOI: 10.1074/jbc.273.49.32870] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
In amino acid-starved yeast cells, inhibition of the guanine nucleotide exchange factor eIF2B by phosphorylated translation initiation factor 2 results in increased translation of GCN4 mRNA. We isolated a suppressor of a mutant eIF2B. The suppressor prevents efficient GCN4 mRNA translation due to inactivation of the small ribosomal subunit protein Rps31 and results in low amounts of mutant 40 S ribosomal subunits. Deletion of one of two genes encoding ribosomal protein Rps17 also reduces the amounts of 40 S subunits but does not suppress eIF2B mutations or prevent efficient GCN4 translation. Our findings show that Rps31-deficient ribosomes are altered in a way that decreases the eIF2B requirement and that the small ribosomal subunit mediates the effects of low eIF2B activity on cell viability and translational regulation in response to eIF2 phosphorylation.
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Affiliation(s)
- P P Mueller
- Institute of Biochemistry and Molecular Biology, University of Berne, CH-3012 Berne, Switzerland.
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37
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Lincoln AJ, Monczak Y, Williams SC, Johnson PF. Inhibition of CCAAT/enhancer-binding protein alpha and beta translation by upstream open reading frames. J Biol Chem 1998; 273:9552-60. [PMID: 9545285 DOI: 10.1074/jbc.273.16.9552] [Citation(s) in RCA: 71] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
CCAAT/enhancer-binding protein (C/EBP) alpha is a bZIP transcription factor whose expression is restricted to specific cell types. Analysis of C/EBPalpha mRNA and protein levels in various mammalian cells indicates that expression of this gene is controlled both transcriptionally and post-transcriptionally. We report here that C/EBPalpha translation is repressed in several cell lines by an evolutionarily conserved upstream open reading frame (uORF), which acts in cis to inhibit C/EBPalpha translation. Mutations that disrupt the uORF completely abolished translational repression of C/EBPalpha. The related c/ebpbeta gene also contains an uORF that suppresses translation. The length of the spacer sequence between the uORF terminator and the ORF initiator codon (7 bases in all c/ebpalpha genes and 4 bases in c/ebpbeta homologs) is precisely conserved. The effects of insertions, deletions, and base substitutions in the C/EBPalpha spacer showed that both the length and nucleotide sequence of the spacer are important for efficient translational repression. Our data indicate that the uORFs regulate translation of full-length C/EBPalpha and C/EBPbeta and do not play a role in generating truncated forms of these proteins, as has been suggested by start site multiplicity models.
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Affiliation(s)
- A J Lincoln
- Advanced BioScience Laboratories-Basic Research Program, NCI-Frederick Cancer Research and Development Center, National Institutes of Health, Frederick, Maryland 21702-1201, USA
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38
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Vivier MA, Lambrechts MG, Pretorius IS. Coregulation of starch degradation and dimorphism in the yeast Saccharomyces cerevisiae. Crit Rev Biochem Mol Biol 1998; 32:405-35. [PMID: 9383611 DOI: 10.3109/10409239709082675] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
Saccharomyces cerevisiae, the exemplar unicellular eukaryote, can only survive and proliferate in its natural habitats through constant adaptation within the constraints of a dynamic ecosystem. In every cell cycle of S. cerevisiae, there is a short period in the G1 phase of the cell cycle where "sensing" transpires; if a sufficient amount of fermentable sugars is available, the cells will initiate another round of vegetative cell division. When fermentable sugars become limiting, the yeast can execute the diauxic shift, where it reprograms its metabolism to utilize nonfermentable carbon sources. S. cerevisiae can also initiate the developmental program of pseudohyphal formation and invasive growth response, when essential nutrients become limiting. S. cerevisiae shares this growth form-switching ability with important pathogens such as the human pathogen, Candida albicans, and the corn smut pathogen Ustilago maydis. The pseudohyphal growth response of S. cerevisiae has mainly been implicated as a means for the yeast to search for nutrients. An important observation made was that starch-degrading S. cerevisiae strains have the added ability to form pseudohyphae and grow invasively into a starch-containing medium. More significantly, it was also shown that the STA1-3 genes encoding three glucoamylase isozymes responsible for starch hydrolysis in S. cerevisiae are coregulated with a gene, MUC1, essential for pseudohyphal and invasive growth. At least two putative transcriptional activators, Mss10p and Mss11p, are involved in this regulation. The Muc1p is a putative integral membrane-bound protein similar to mammalian mucin-like proteins that have been implicated in the ability of cancer cells to invade other tissues. This provided us with an excellent example of integrative control between nutrient sensing, signaling, and differential development.
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Affiliation(s)
- M A Vivier
- Institute for Wine Biotechnology, University of Stellenbosch, South Africa
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39
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Ruiz-Echevarría MJ, González CI, Peltz SW. Identifying the right stop: determining how the surveillance complex recognizes and degrades an aberrant mRNA. EMBO J 1998; 17:575-89. [PMID: 9430648 PMCID: PMC1170407 DOI: 10.1093/emboj/17.2.575] [Citation(s) in RCA: 100] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
The nonsense-mediated mRNA decay (NMD) pathway functions by checking whether translation termination has occurred prematurely and subsequently degrading the aberrant mRNAs. In Saccharomyces cerevisiae, it has been proposed that a surveillance complex scans 3' of the premature termination codon and searches for the downstream element (DSE), whose recognition by the complex identifies the transcript as aberrant and promotes its rapid decay. The results presented here suggest that translation termination is important for assembly of the surveillance complex. Neither the activity of the initiation ternary complex after premature translation termination has occurred nor the elongation phase of translation are essential for the activity of the NMD pathway. Once assembled, the surveillance complex is active for searching and recognizing a DSE for approximately 200 nt 3' of the stop codon. We have also identified a stabilizer sequence (STE) in the GCN4 leader region that inactivates the NMD pathway. Inactivation of the NMD pathway, as a consequence of either the DSE being too far from a stop codon or the presence of the STE, can be circumvented by inserting sequences containing a new translation initiation/termination cycle immediately 5' of the DSE. Further, the results indicate that the STE functions in the context of the GCN4 transcript to inactivate the NMD pathway.
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Affiliation(s)
- M J Ruiz-Echevarría
- Department of Molecular Genetics and Microbiology, Robert Wood Johnson Medical School-UMDNJ, Piscataway, NJ 08854, USA
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40
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Hinnebusch AG. Translational regulation of yeast GCN4. A window on factors that control initiator-trna binding to the ribosome. J Biol Chem 1997; 272:21661-4. [PMID: 9268289 DOI: 10.1074/jbc.272.35.21661] [Citation(s) in RCA: 437] [Impact Index Per Article: 16.2] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Affiliation(s)
- A G Hinnebusch
- Laboratory of Eukaryotic Gene Regulation, NICHD, National Institutes of Health, Bethesda, Maryland 20892, USA
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41
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Abstract
Translation processes in plants are very similar to those in other eukaryotic organisms and can in general be explained with the scanning model. Particularly among plant viruses, unconventional mRNAs are frequent, which use modulated translation processes for their expression: leaky scanning, translational stop codon readthrough or frameshifting, and transactivation by virus-encoded proteins are used to translate polycistronic mRNAs; leader and trailer sequences confer (cap-independent) efficient ribosome binding, usually in an end-dependent mechanism, but true internal ribosome entry may occur as well; in a ribosome shunt, sequences within an RNA can be bypassed by scanning ribosomes. Translation in plant cells is regulated under conditions of stress and during development, but the underlying molecular mechanisms have not yet been determined. Only a small number of plant mRNAs, whose structure suggests that they might require some unusual translation mechanisms, have been described.
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Affiliation(s)
- J Fütterer
- Institute of Plant Sciences, ETHZ, Zürich, Switzerland
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42
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Reynolds K, Zimmer AM, Zimmer A. Regulation of RAR beta 2 mRNA expression: evidence for an inhibitory peptide encoded in the 5'-untranslated region. J Cell Biol 1996; 134:827-35. [PMID: 8769409 PMCID: PMC2120959 DOI: 10.1083/jcb.134.4.827] [Citation(s) in RCA: 60] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023] Open
Abstract
Regulation of mRNA translation and stability plays an important role in the control of gene expression during embryonic development. We have recently shown that the tissue-specific expression of the RAR beta 2 gene in mouse embryos is regulated at the translational level by short upstream open reading frames (uORFs) In the 5'-untranslated region (Zimmer, A., A.M. Zimmer, and K. Reynolds. 1994. J. Cell Biol. 127:1111-1119). To gain insight into the molecular mechanism, we have performed a systematic mutational analysis of the uORFs. Two series of constructs were tested: in one series, each uORF was individually inactivated by introducing a point mutation in its start codon; in the second series, all but one ORF were inactivated. Our results indicate that individual uORFs may have different functions. uORF4 seems to inhibit translation of the major ORF in heart and brain, while uORFs 2 and 5 appear to be important for efficient translation in all tissues. To determine whether the polypeptide encoded by uORF4 or the act of translating it, is the significant event, we introduced point mutations to create silent mutations or amino acid substitutions in uORF4. Our results indicate that the uORF4 amino acid coding sequence is important for the inhibitory effect on translation of the downstream major ORF.
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Affiliation(s)
- K Reynolds
- Unit of Developmental Biology, National Institute of Mental Health, Bethesda, Maryland 20892-4090, USA
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43
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Tavernarakis N, Thireos G. Genetic evidence for functional specificity of the yeast GCN2 kinase. MOLECULAR & GENERAL GENETICS : MGG 1996; 251:613-8. [PMID: 8709969 DOI: 10.1007/bf02173652] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
In yeast the GCN2 kinase mediates translational control of GCN4 by phosphorylating the alpha subunit of eIF-2 in response to extracellular amino acid limitation. Although phosphorylation of eIF-2 alpha has been shown to inhibit global protein synthesis, amino acid starvation results in a specific activation effect on GCN4 mRNA translation. Under the same conditions, translation of other mRNAs appears only slightly affected. The mechanism responsible for the observed selectivity of the GCN2 kinase is not clear. Here, we present genetic evidence that suggests that locally restricted action of the GCN2 kinase facilitates GCN4-specific translational regulation.
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Affiliation(s)
- N Tavernarakis
- Institute of Molecular Biology and Biotechnology, Foundation for Research and Technology, Heraklion, Crete, Greece
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Laine RO, Hutson RG, Kilberg MS. Eukaryotic gene expression: metabolite control by amino acids. PROGRESS IN NUCLEIC ACID RESEARCH AND MOLECULAR BIOLOGY 1996; 53:219-48. [PMID: 8650304 DOI: 10.1016/s0079-6603(08)60146-4] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
Our understanding of the metabolite control in mammalian cells lags far behind that in prokaryotes. This is particularly true for amino-acid-dependent gene expression. Few proteins have been identified for which synthesis is selectively regulated by amino-acid availability, and the mechanisms for control of transcription and translation in response to changes in amino-acid availability have not yet been elucidated. The intimate relationship between amino-acid supply and the fundamental cellular process of protein synthesis makes amino-acid-dependent control of gene expression particularly important. Future studies should provide important insight into amino-acid and other nutrient signaling pathways, and their impact on cellular growth and metabolism.
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Affiliation(s)
- R O Laine
- Department of Biochemistry and Molecular Biology, University of Florida College of Medicine, Gainesville 32610, USA
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Grant CM, Miller PF, Hinnebusch AG. Sequences 5' of the first upstream open reading frame in GCN4 mRNA are required for efficient translational reinitiation. Nucleic Acids Res 1995; 23:3980-8. [PMID: 7479046 PMCID: PMC307320 DOI: 10.1093/nar/23.19.3980] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023] Open
Abstract
Translation of yeast GCN4 mRNA occurs by a reinitiation mechanism that is modulated by amino acid levels in the cell. Ribosomes which translate the first of four upstream open reading frames (uORFs) in the mRNA leader resume scanning and can reinitiate downstream. Under non-starvation conditions reinitiation occurs at one of the remaining three uORFs and GCN4 is repressed. Under starvation conditions, in contrast, ribosomes bypass the uORFs and reinitiate at GCN4 instead. The high frequency of reinitiation following uORF1 translation depends on an adequate distance to the next start codon and particular sequences surrounding the uORF1 stop codon. We present evidence that sequences 5' to uORF1 also strongly enhance reinitiation. First, reinitiation was severely inhibited when uORF1 was transplanted into the position of uORF4, even though the native sequence environment of the uORF1 stop codon was maintained, and this effect could not be accounted for by the decreased uORF1-GCN4 spacing. Second, insertions and deletions in the leader preceding uORF1 greatly reduced reinitiation at GCN4. Sequences 5' to uORF1 may influence the probability of ribosome release following peptide termination at uORF1. Alternatively, they may facilitate rebinding of an initiation factor required for reinitiation prior to resumption of the scanning process.
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Affiliation(s)
- C M Grant
- Section on Molecular Genetics of Lower Eukaryotes, National Institute of Child Health and Human Development, Bethesda, MD 20892, USA
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Luo Z, Freitag M, Sachs MS. Translational regulation in response to changes in amino acid availability in Neurospora crassa. Mol Cell Biol 1995; 15:5235-45. [PMID: 7565672 PMCID: PMC230771 DOI: 10.1128/mcb.15.10.5235] [Citation(s) in RCA: 51] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Abstract
We examined the regulation of Neurospora crassa arg-2 and cpc-1 in response to amino acid availability.arg-2 encodes the small subunit of arginine-specific carbamoyl phosphate synthetase; it is subject to unique negative regulation by Arg and is positively regulated in response to limitation for many different amino acids through a mechanism known as cross-pathway control. cpc-1 specifies a transcriptional activator important for crosspathway control. Expression of these genes was compared with that of the cytochrome oxidase subunit V gene, cox-5. Analyses of mRNA levels, polypeptide pulse-labeling results, and the distribution of mRNA in polysomes indicated that Arg-specific negative regulation of arg-2 affected the levels of both arg-2 mRNA and arg-2 mRNA translation. Negative translational effects on arg-2 and positive translational effects on cpc-1 were apparent soon after cells were provided with exogenous Arg. In cells limited for His, increased expression of arg-2 and cpc-1, and decreased expression of cox-5, also had translational and transcriptional components. The arg-2 and cpc-1 transcripts contain upstream open reading frames (uORFs), as do their Saccharomyces cerevisiae homologs CPA1 and GCN4. We examined the regulation of arg-2-lacZ reporter genes containing or lacking the uORF start codon; the capacity for arg-2 uORF translation appeared critical for controlling gene expression.
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Affiliation(s)
- Z Luo
- Department of Chemistry, Biochemistry, and Molecular Biology, Oregon Graduate Institute of Science and Technology, Portland, Oregon 97291-1000, USA
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Chu SC, Wu HP, Banks TC, Eissa NT, Moss J. Structural diversity in the 5'-untranslated region of cytokine-stimulated human inducible nitric oxide synthase mRNA. J Biol Chem 1995; 270:10625-30. [PMID: 7537735 DOI: 10.1074/jbc.270.18.10625] [Citation(s) in RCA: 50] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023] Open
Abstract
Inducible nitric oxide synthase, the critical enzyme responsible for the enhanced synthesis of nitric oxide in inflammatory states, is widely expressed in mammalian cells. To evaluate potential regulatory roles of the 5'-untranslated region (UTR) in the human inducible nitric oxide synthase gene, the transcription initiation sites and structure of the 5'-UTR of human inducible nitric oxide synthase were examined. Freshly isolated human alveolar macrophages, bronchial epithelial cells, and several types of cultured cells were evaluated following stimulation with cytokines (i.e. interferon-gamma, interleukin-1 beta, tumor necrosis factor-alpha, and interleukin-6). The mRNA was analyzed by reverse transcription-polymerase chain reaction. Northern analysis, and 5'-rapid amplification of cDNA ends. Despite the presence of a TATA box in the promoter region, multiple transcription initiation sites were observed, some extending several hundred base pairs upstream from the main TATA-directed initiation site. Alternative splicing in the 5'-UTR of human inducible nitric oxide synthase mRNA resulted in further diversity. The TATA-independent inducible nitric oxide synthase mRNA transcripts were up-regulated by cytokines. The long and complex 5'-UTRs contain eight partially overlapping open reading frames upstream of the putative inducible nitric oxide synthase ATG, which may have an important role in translational regulation of human inducible nitric oxide synthase mRNA.
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Affiliation(s)
- S C Chu
- Pulmonary-Critical Care Medicine Branch, NHLBI, National Institutes of Health, Bethesda, Maryland 20892, USA
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Donzé O, Damay P, Spahr PF. The first and third uORFs in RSV leader RNA are efficiently translated: implications for translational regulation and viral RNA packaging. Nucleic Acids Res 1995; 23:861-8. [PMID: 7708504 PMCID: PMC306771 DOI: 10.1093/nar/23.5.861] [Citation(s) in RCA: 45] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Abstract
Rous sarcoma virus (RSV) RNA leader contains three short upstream open reading frames. We have shown recently that both uORFs 1 and 3 influence in vivo translation of the downstream gag gene and are involved in the virus RNA packaging process. In this report, we have studied the translational events occurring at the upstream AUGs in vivo. We show that (i) the first and third AUGs are efficient translational initiation sites; (ii) ribosomes reinitiate efficiently at AUG3; and (iii) deletions in the intercistronic distance between uORF1 and 3 (which is well conserved among avian strains) prevent ribosome initiation at AUG3, thus increasing translation efficiency at the downstream AUGgag. The roles of the uORFs in translation and packaging are discussed.
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Affiliation(s)
- O Donzé
- Department of Molecular Biology, University of Geneva, Switzerland
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Multicopy tRNA genes functionally suppress mutations in yeast eIF-2 alpha kinase GCN2: evidence for separate pathways coupling GCN4 expression to unchanged tRNA. Mol Cell Biol 1994. [PMID: 7969132 DOI: 10.1128/mcb.14.12.7920] [Citation(s) in RCA: 27] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
GCN2 is a protein kinase that stimulates translation of GCN4 mRNA in amino acid-starved cells by phosphorylating the alpha subunit of translation initiation factor 2 (eIL-2). We isolated multicopy plasmids that overcome the defective derepression of GCN4 and its target genes caused by the leaky mutation gcn2-507. One class of plasmids contained tRNA(His) genes and conferred efficient suppression only when cells were starved for histidine; these plasmids suppressed a gcn2 deletion much less efficiently than they suppressed gcn2-507. This finding indicates that the reduction in GCN4 expression caused by gcn2-507 can be overcome by elevating tRNA(His) expression under conditions in which the excess tRNA cannot be fully aminoacylated. The second class of suppressor plasmids all carried the same gene encoding a mutant form of tRNA(Val) (AAC) with an A-to-G transition at the 3' encoded nucleotide, a mutation shown previously to reduce aminoacylation of tRNA(Val) in vitro. In contrast to the wild-type tRNA(His) genes, the mutant tRNA(Val) gene efficiently suppressed a gcn2 deletion, and this suppression was independent of the phosphorylation site on eIF-2 alpha (Ser-51). Overexpression of the mutant tRNA(Val) did, however, stimulate GCN4 expression at the translational level. We propose that the multicopy mutant tRNA(Val) construct leads to an accumulation of uncharged tRNA(Val) that derepresses GCN4 translation through a pathway that does not involve GCN2 or eIF-2 alpha phosphorylation. This GCN2-independent pathway was also stimulated to a lesser extent by the multicopy tRNA(His) constructs in histidine-deprived cells. Because the mutant tRNA(Val) exacerbated the slow-growth phenotype associated with eIF-2 alpha hyperphosphorylation by an activated GCN2c kinase, we suggest that the GCN2-independent derepression mechanism involves down-regulation of eIF-2 activity.
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Vazquez de Aldana CR, Wek RC, Segundo PS, Truesdell AG, Hinnebusch AG. Multicopy tRNA genes functionally suppress mutations in yeast eIF-2 alpha kinase GCN2: evidence for separate pathways coupling GCN4 expression to unchanged tRNA. Mol Cell Biol 1994; 14:7920-32. [PMID: 7969132 PMCID: PMC359331 DOI: 10.1128/mcb.14.12.7920-7932.1994] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023] Open
Abstract
GCN2 is a protein kinase that stimulates translation of GCN4 mRNA in amino acid-starved cells by phosphorylating the alpha subunit of translation initiation factor 2 (eIL-2). We isolated multicopy plasmids that overcome the defective derepression of GCN4 and its target genes caused by the leaky mutation gcn2-507. One class of plasmids contained tRNA(His) genes and conferred efficient suppression only when cells were starved for histidine; these plasmids suppressed a gcn2 deletion much less efficiently than they suppressed gcn2-507. This finding indicates that the reduction in GCN4 expression caused by gcn2-507 can be overcome by elevating tRNA(His) expression under conditions in which the excess tRNA cannot be fully aminoacylated. The second class of suppressor plasmids all carried the same gene encoding a mutant form of tRNA(Val) (AAC) with an A-to-G transition at the 3' encoded nucleotide, a mutation shown previously to reduce aminoacylation of tRNA(Val) in vitro. In contrast to the wild-type tRNA(His) genes, the mutant tRNA(Val) gene efficiently suppressed a gcn2 deletion, and this suppression was independent of the phosphorylation site on eIF-2 alpha (Ser-51). Overexpression of the mutant tRNA(Val) did, however, stimulate GCN4 expression at the translational level. We propose that the multicopy mutant tRNA(Val) construct leads to an accumulation of uncharged tRNA(Val) that derepresses GCN4 translation through a pathway that does not involve GCN2 or eIF-2 alpha phosphorylation. This GCN2-independent pathway was also stimulated to a lesser extent by the multicopy tRNA(His) constructs in histidine-deprived cells. Because the mutant tRNA(Val) exacerbated the slow-growth phenotype associated with eIF-2 alpha hyperphosphorylation by an activated GCN2c kinase, we suggest that the GCN2-independent derepression mechanism involves down-regulation of eIF-2 activity.
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Affiliation(s)
- C R Vazquez de Aldana
- Section on Molecular Genetics of Lower Eukaryotes, National Institute of Child Health and Human Development, Bethesda, Maryland 20892
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